The present invention relates to telephony systems and, more particularly, to speakerphone circuits which interface with telephone lines to provide voice-switched speech communication.
Prior art integrated speakerphone circuits are well known. For instance, the MC34018 Speakerphone Circuit manufactured by Motorola Inc. provides voice-switched half-duplex communication. Prior art speakerphones comprise both transmit and receive signal paths each of which include an attenuator in series with the signal path. The transmit and receive attenuators operate in a complementary manner, i.e., while one is at maximum gain the other is at maximum attenuation.
Typically, control of the respective gains of the transmit and receive attenuators is accomplished by comparing the amplitude of the transmit signal appearing at the output of the transmit attenuator to the receive signal appearing at the input of the receive attenuator and controlling the level of a control voltage differentially supplied to the attenuators. Hence, if a far-end talker is speaking the receive signal will be greater than the transmit signal and the transmit attenuator will be at maximum loss while the receive attenuator is at maximum gain. This is referred to as the receive mode of operation. Conversely, if the near-end talker is speaking the transmit signal will be greater than the receive signal and the gain of the transmit attenuator is maximized while that of the receive attenuator is minimized. In this mode the speakerphone is in a transmit mode. Finally, if neither talker is speaking, the control voltage goes to an idle mode value which sets the gains of the two attenuators to an equal mid-value setting to allow the first person speaking to set the speakerphone in the correct mode of operation.
Because comparison of the transmit and receive signals occurs after the transmit attenuator it is very difficult for the near-end talker to break-in on the far-end talker since the transmit attenuator is at maximum loss. The same is true even if the far-end talker has paused.
Hence, a need exists for a high quality speakerphone having fast sensitive switching between the receive and transmit modes.